Don't worry too much about the ' 3 second' leak test. Remember the air can also escape past the piston seal ,if it is a poor fit,just as much as it can leak past the loading tap. But.....when you actualy fire the rifle the piston seal will flare slightly to improve the seal and ,as you have guessed, due to the speed that the firing cycle happens I doubt that a slight leak at the tap will affect velocity very much. You can improve the tap leakage by coating it in thick grease before insertion.
I honestly believe that your rifle is oversprung , so in the absense of a softer option you can try shortening yours a coil at a time. The problem with that is........shortening a spring makes it stiffer but it does reduce preload, so worth a try until you can find a softer spring.
Pete.